Although some embryological and morphological features speak in favour of a neuronal character of rodent pinealocytes, histochemistry and ultrastructure let this issue appear controversial. Using antibodies to different neurofilaments, the neural adhesion molecule L1, synaptophysin and tubulin as neuronal markers, the pineal glands of rat and guinea-pig were studied by means of immunofluorescence. Neurofilament-immunoreactivity was present in some rat pineal nerve fibers and in the majority of guinea-pig pinealocytes, L1 decorated rat intrapineal nerve fibers, synaptophysin was almost ubiquitously distributed in the pineal of both species, while tubulin-immunofluorescence was seen in nerve fibers of rat and guinea-pig pineal and in some pinealocytes of the latter. These findings speak in favour of the neuronal character of guinea-pig pinealocytes. The lack of neurofilament- and tubulin-immunoreactivity in rat pinealocytes might be attributable to very low concentrations of these proteins or species differences as to their expression. Further studies including in situ-hybridisation of relevant mRNAs will be necessary to answer these questions definitely.